Emergent Aquatic Insects: Assemblage Structure and Patterns of Availability in Freshwater Wetlands of the Lower Columbia River Estuary

Transcription

1 Emergent Aquatic Insects: Assemblage Structure and Patterns of Availability in Freshwater Wetlands of the Lower Columbia River Estuary Mary Frances Ramirez A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science University of Washington 2008 Program Authorized to Offer Degree: School of Aquatic and Fishery Sciences

2 University of Washington Graduate School This is to certify that I have examined this copy of a master s thesis by Mary Frances Ramirez and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Committee Members: Charles Simenstad Dan Bottom Claire Horner-Devine

3 Date In presenting this thesis in partial fulfillment of the requirements for a master s degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this thesis is allowable only for scholarly purposes, consistent with fair use as prescribed in the U.S. Copyright Law. Any other reproduction for any purposes or by any means shall not be allowed without my written permission. Signature Date

4 TABLE OF CONTENTS Page List of Figures... ii List of Tables... iii Chapter 1. Introduction...1 Study Objectives...7 Chapter 2. Spatial and Temporal Patterns in Insect Emergence...10 Introduction...10 Methods...13 Results...16 Discussion...20 Chapter 3. Insect Emergence and Export...45 Introduction...45 Methods...48 Results...52 Discussion...55 Chapter 4. Final Synthesis...81 References...85 Appendix A Emergent insect densities...96 Appendix B Proportional composition by trap type...98 Appendix C SIMPER results of dissimilarity between microhabitats...99 Appendix D SIMPER results of similarity within microhabitats Appendix E Chironomid genera densities Appendix F. Sample site metrics Appendix G Emergent insect densities Appendix H Chironomid genera denisities Appendix I Export quantities standardized by site metrics i

5 LIST OF FIGURES Figure Number Page 1.1. The four life stages of chironomids Map of 2006 sample locations at Russian Island, Columbia River estuary Emergent insect composition and daily density, Russian Island NMDS plot of emergent insect abundance data coded by sample month, Monthly density of insects collected by emergent and fallout traps, Daily density by microhabitat, Proportional occurrence by microhabitat of common emergent insects, NMDS plots of emergent insect abundance data coded by microhabitat, Densities of the two dominant chironomid subfamilies, Temporal distribution of daily chironomid genera densities, Proportional chironomid emergence by microhabitat, NMDS plot with dominant chironomid densities, Area curves for emergent insect and chironomid densities, Shannon-Wiener diversity values, Map of 2007 sampling sites, Columbia River estuary Emergent abundance trends, 2007 diel study Daily emergent densities of select insects, Daily emergent densities of major insect families, Shannon-Wiener diversity values for emergent insects, Daily emergent densities of major chironomid taxa, Shannon-Wiener diversity values for chironomid genera, Composition of export samples, Standardized counts of export samples, Correlation between total chironomid export and emergence, Probability of export by sample month, Daily densities of major chironomid genera collected in 2006 and Neuston sample composition and quantity from distributary sloughs, ii

6 LIST OF TABLES Table Number Page 2.1. ANOSIM results for assemblage differences by microhabitat, SIMPER results for chironomid taxa by microhabitat, Seasonal counts of emergence integrated over sample period, Examples of annual insect emergence from wetland systems Sum of emergent insect densities over seasonal sample events, ANOSIM results for assemblage differences by site, Sum of emergent chironomid densities over seasonal sample events, SIMPER results for chironomid taxa by site, Counts of taxa collected in export samples by site, iii

7 ACKNOWLEDGEMENTS I offer my sincere thanks to all those who supported me throughout this process. I am very grateful for the guidance and advice from my thesis committee, Charles Si Simenstad, Dan Bottom, and Claire Horner-Devine. I could not have completed this work without the invaluable help from the entire Wetland Ecosystem Team, especially Sarah Spilseth, Jeff Cordell, Jason Toft, Jennifer Burke, Beth Armbrust, and Claire Levy. Additionally, I offer many thanks to Lia Stamatiou for all her assistance in the field and lab. From how to pull start a dead boat to the local lore of Astoria, I have truly learned a lot working with Lia. I am also very appreciative to Susan Hinton, George McCabe, and Regan McNatt from NOAA Fisheries, for teaching me about the estuary and working in the mud. Leonard Ferrington, with the University of Minnesota Chironomid Research Group, graciously offered to verify some of my chironomid identifications and advise me in the learning stages of this difficult task. Financial support for my research and education came from the U.S. Army Corps of Engineers, Bonneville Power Administration, and the School of Aquatic and Fishery Sciences. Finally, I am incredibly grateful for all the support and encouragement over the past few years from my family and friends. iv

8 Chapter 1. Introduction 1 Estuarine ecosystems often are characterized by high primary and secondary productivity, allowing them to support a great diversity of fish and invertebrate species (Teal 1962, Odum 1980, Kneib 1997, Simenstad and Cordell. 2000, Beck et al. 2001). Tidal marshes in particular, produce large amounts of new plant tissue annually, which is then converted by bacterial decomposition and physical breakdown into organic detritus. In some estuarine systems marsh-derived detritus is a major food source for the majority of consumers (Teal 1962), including abundant epibenthic crustaceans and diverse insect assemblages. Wetland insects play a prominent role in the consumption and processing of primary production and associated detritus and serve as an important food source for higher trophic levels, including a large number of fish, invertebrate, and avian species (Davies 1984, Stagliano et al. 1998). Here I investigate the composition of emergent insects and their availability to juvenile salmon (Oncorhynchus spp.) in tidal wetlands of the Columbia River estuary. Recently, there has been an emerging recognition in the uniformity of wetland insects in the diets of juvenile Chinook salmon (O. tshawytscha) rearing in estuaries of the Pacific Northwest. There is, however, a lack of knowledge in the assemblage structure, particularly within the Chironomidae (Diptera) family, and fine-scale distribution of these emergent aquatic insects. Studying the patterns and processes of their spatial and temporal distribution within tidal channel environments provides important information on the availability of this essential prey resource to juvenile Chinook salmon. Additionally, changes in the composition of an insect community occur relatively quickly in time and space, making the group useful in monitoring the ecological condition of an area simply by examining the insects that live there. In this study, I focus on identifying patterns of insect emergence from tidal wetlands of the Columbia River estuary. I was interested in how assemblages were structured, with a particular emphasis on the chironomid family, and how the taxonomic composition changed over both spatial and temporal scales. Further, I address questions on the availability of these insects to juvenile Chinook salmon rearing in the estuary.

9 2 Availability may be enhanced in areas through the movement of wetland insects across aquatic boundaries, potentially serving as a subsidized resource in open water habitats away from where the insect was produced. Alternatively, their availability to juvenile Chinook may be limited through such factors as synchronized daily patterns in emergence or associations with inaccessible microhabitats. The work presented here on aquatic insect emergence and availability is nested within the larger research led by the National Oceanic and Atmospheric Administration (NOAA) and University of Washington on population dynamics and habitat use of juvenile Chinook salmon in the freshwater tidal marshes of the Columbia River. The importance of Northwest estuaries as nursery habitat for juvenile salmon and other nekton is well established in the literature (Healey 1982, Levy and Northcote 1982, Simenstad et al. 1982, Kneib 1997, Beck et al. 2001). In addition to providing a connection in the landscape between freshwater spawning and rearing grounds and the nutrient rich marine waters of the Pacific Ocean, estuaries reportedly offer abundant feeding opportunities, refuge from marine predators, and a physiological transition zone where juvenile fish may acclimate to saline waters (Simenstad et al. 1982, Thorpe 1994, Bottom et al. 2005). This study developed in response to questions related to this specific aspect of abundant foraging opportunities, with numerous previous reports highlighting the importance of wetland insects in the diets of juvenile Chinook salmon during their migration through and residency in many coastal estuaries (Simenstad et al. 1982) including those of the Fraser River (Levy et al. 1979, Levy and Northcote 1982), Puyallup River (Shreffler et al. 1992), Salmon River (Gray et al. 2002), and Columbia River (Lott 2004). Among wetland insects consumed by juvenile Chinook, the Chironomidae, a family of small aquatic flies, compose a large proportion, both numerically and gravimetrically, of the diets of fish caught within a variety of wetland ecosystem types (Simenstad and Cordell 2000, Lott 2004, Gray 2005). Indices of Relative Importance (IRI) were shown to reach as high as 90% in an emergent marsh of the Columbia River estuary for the chironomid family (Lott 2004). The IRI is a compound index that describes the relative contribution of stomach contents to the diet (Pinkas et al. 1971).

10 3 This index is often used in many diet studies and facilitates comparisons among food types (Cortes 1997). Additionally, Lott (2004) noted the significant observation of the apparent targeting by juvenile Chinook salmon of the specific emergent life stage in chironomids from tidal marsh channels (Lott 2004). As opposed to feeding on adults floating on the water s surface, this behavior directly links the predator to the prey s habitat of origin. Recent studies, however, have expanded our understanding of the importance of wetland insects by demonstrating their contribution to consumers beyond their site of origin. These insects, again particularly chironomids and other dipteran flies, constitute at times the majority of prey in the diets of juvenile Chinook caught in open water habitats, for example along mainstem beaches and in large distributary sloughs of the Columbia River estuary (Bottom et al. 2008). Furthermore, an earlier study from the Fraser River estuary, found chironomids in 31% of juvenile Chinook stomachs caught up to 300 m offshore (Whitehouse et al. 1993). Such findings demonstrate the widespread significance of wetland insects throughout the broader range of these estuarine systems. Chironomidae Chironomids, commonly known as midges, contribute to the food of an extensive range of both small and large predators throughout all stages of their life cycle (Armitage 1995), and are especially important to waterfowl and waders (King and Wrubleski 1998, Einarsson and Gardarsson 2004, Smith et al. 2007) and a vast number of fish species (Mundie 1971, Jackson and Fisher 1986, Merz 2002, Lott 2004). The family is regularly reported as the dominant insect group from wetland (Wrubleski 1987, Leeper and Taylor 1998, Stagliano et al. 1998, Whiles and Goldowitz 2001, MacKenzie and Kaster 2004) and estuarine (Williams and Williams 1998a, Strayer and Smith 2000, MacKenzie 2005) systems. They are the most widespread of all aquatic insect families, occurring on all continents (Ferrington 2008). Chironomids are also exceptionally diverse, with estimates as high as 15,000 species, and select taxa within the family are identified as able to tolerate most climates and conditions, including a wide array of water quality (Cranston

11 4 1995, Ferrington 2008). Their high species richness and ubiquitous nature makes this family particularly useful in the biomonitoring of aquatic ecosystems (Waite et al. 2004). Despite the significant abundance and richness associated with the chironomid family, or possibly because of it, the group is often neglected in biological surveys (Armitage 1995, Williams and Williams 1998a, Reynolds and Benke 2006). Their small size, superficial similarity, and high diversity make chironomids tempting to overlook and treat at broad taxonomic levels. By doing so, researchers inhibit their ability to infer ecological relationships, document changes in the community, and make adequate comparisons between systems. Within the Pacific Northwest, the chironomid fauna is still relatively poorly known, with very little published on the area except the papers of Morley and Ring (1972a, 1972b). Aquatic Insect Emergence The targeting by juvenile Chinook of the emergent life stage in chironomids is a selection for insects undergoing a dramatic transition in their life cycle. This emergence, or metamorphosis, is a switch from their larval period of feeding and growth to the reproductive and dispersal stage of the adult form. From a developmental perspective, Chironomids, as well as many other aquatic insects, are holometabolous, resulting in four distinct life stages: egg, larva, pupa, and adult (Fig. 1.1). The larval stage consists of an active benthic period, followed by the short pupal phase, also benthic, where the adult characteristics develop (Oliver 1971). In the last stage of their life cycle, the insect undergoes both a metamorphosis to the adult form and a transition from the aquatic to the terrestrial environment (Davies 1984). After reaching maturity, the pupae begin the process of emergence where they move to the surface of the water, either through the water column or by crawling along vegetation. There is a short period of waiting at the water surface while their wings dry and open completely, after which the insect will emerge and fly away as an adult. It is during the emergence process, as the insect moves upward through the water column, that it is most susceptible to predation (Oliver 1971). Lott (2004) describes this particular stage as the dominant form appearing in juvenile

12 5 Chinook diets, identified from such things as the wings or legs not yet fully extended or hardened. Insect emergence is not a random occurrence, but rather is seen to display a relationship with environmental variables such as, temperature, photoperiod, water level, and oxygen concentration (Neumann and Kruger 1985, Armitage 1995). Emergence often occurs over a particular time of year depending on conditions, though overall patterns are derived from the more specialized individual timing of component species. Many aquatic insect species emerge in a highly synchronized fashion at a distinct time each season. This pattern may benefit the insects by maximizing chances of mating success while minimizing individual losses through a brief saturation of the predator community (Davies 1984). Daily rhythms in emergence have also been shown in the laboratory as well as many natural systems and have been attributed to changes in light intensity and/or water temperature (Neumann 1986, Armitage 1995). In addition to distinctions in time, habitat features, such as substrate size and stability, vegetation, and water velocity, may further localize the emergence of some species (Davies 1984). The emergence of adult insects from aquatic habitats represents an important export of nutrients and energy from one environment to the other and is a unique mechanism linking terrestrial and aquatic ecosystems (Jackson and Fisher 1986, Nakano and Murakami 2001, Power et al. 2004, Baxter et al. 2005). The cross habitat effects may be significant, with terrestrial consumers often shown to aggregate near stream banks at periods of peak insect emergence to take advantage of this resource subsidy (Sabo and Power 2002, Baxter et al. 2005). However, even within aquatic environments, the downstream drift of insects in water currents represents an additional mechanism for transporting invertebrate prey to consumers away from where they were produced (Brittain and Eikland 1988). This movement may affect insects in the process of emergence as well as adults that are deposited on the water surface. In view of estuaries, Odum (1980) identified the daily flushing by tides as the primary physical agent for facilitating material transportation, but river flows also act as a mechanism for distributing nutrients and organisms throughout these environments (Dame and Allen 1996).

13 6 The Columbia River Estuary The Columbia River estuary is located along the boundary between the states of Washington and Oregon in the Pacific Northwest, representing the terminus of the extensive 660,000 km 2 river basin. The estuary is defined as encompassing the entire habitat continuum from river mouth to Bonneville Dam wherein tidal forces and river flow interact. This project includes three sites within the freshwater tideland reach: Russian Island within Cathlamet Bay (Rkm 35), Wallace Island (Rkm 77), and Lord Island (Rkm 101). The surge plain of this region is of low relief, typically experiencing flooding at high tide as water permeates numerous dendritic tidal streams. Despite their apparent ecological significance, estuaries have experienced dramatic loss and degradation of salmonid habitat. The Columbia River estuary has undergone significant changes following the rapid growth of human civilization. Damming, dredging, diking, and the development of irrigation in the basin, among other landscape altering activities, have resulted in the loss of a variety of ecosystems. The loss of habitat is extensive in the lower estuary, particularly in emergent and forested wetlands that historically may have supported salmon with estuarine-resident life histories (Burke 2005, Bottom et al. 2005). An estimated 77% of tidal marshes and 62% of swamps have been lost in relation to the historic total existing prior to 1870 (Thomas 1983). Diking and the regulation of river-flow timing and magnitude have limited the extent of tidal inundation in many wetland habitats and left the river disconnected from its floodplain (Bottom et al. 2005). Energy associated with such periodic disruptions had promoted and sustained highly productive habitats, providing the base of the detrital food chain that supports salmonid production. The current disconnect in the estuarine landscape limits the import of organic matter to the system that likely occurred historically during seasonal freshets and winter floods (Bottom et al. 2005). This loss in habitat and connectivity is coupled with significant declines in salmon populations and life history types that had presumably evolved through a dependence on the continuity of these shallow wetland systems (Rich 1920, Simenstad et al. 2000, Bottom et al. 2005). Since 1991, 13 Columbia River salmon stocks have been listed as threatened or

14 endangered under the U.S. Endangered Species Act (Bottom et al. 2005). With these listings has come a pressing interest in habitat restoration for salmon conservation. 7 Juvenile Salmon in Estuaries Within the freshwater tidal zone, juvenile salmon utilize the many distributary and dendritic channels that provide areas of subdued velocity, abundant feeding opportunities, and low predation pressure (Bottom et al. 2005). This is an important transition point in the landscape wherein species may reside for days to weeks to adjust physiologically to the changing environment (Simenstad and Cordell 2000). Patterns of habitat use and duration of residence vary among Pacific salmon species as a variety of strategies have been developed to utilize the estuary. Ocean-type Chinook are considered the most estuarine dependent of the salmon species, moving downstream immediately or soon after emergence and commonly rearing for several weeks within coastal wetland systems (Healey 1982, Simenstad et al. 1982, Levings 1994). Historically, Columbia River Chinook exhibited a diversity of life history strategies, seemingly benefiting from a series of streams, rivers and the estuary as rearing habitat throughout different stages in their life cycle (Rich 1920, Burke 2005). Research surveys have documented the consistent seasonal use of the Columbia River s diverse freshwater tidal wetlands by juvenile Chinook salmon, despite periods of channel dewatering on low tidal cycles (Lott 2004, Bottom et al. 2005). On the flooding tide, juveniles move into the marshes along tidal creeks, and then retreat to nearby areas that do retain water on the ebb. Study Objectives The primary objective of this study was to provide new data on the spatial and temporal distribution of emergent insect assemblages in tidal wetlands of the Columbia River estuary. To do so, I examined patterns under different scales of observation. One, the level of taxonomy, with chironomids treated at the family and generic level, which also provided a baseline record of the previously unstudied chironomid fauna. Second, I observed emergence patterns under different spatial and temporal resolutions, from

15 8 seasonal trends to daily rhythms, and from microhabitats to ecosystems. In association with researching the processes of insect emergence, I described the extent of prey export from three tidal channels to adjacent distributary sloughs and beyond. This movement across aquatic boundaries implies the potential of wetland prey to serve as a subsidized food resource to juvenile Chinook foraging in open water habitats of the broader estuary. The following chapters are organized around the different study components, beginning with a survey I conducted in 2006 of insect emergence from Russian Island, a freshwater emergent marsh within the lower Columbia River estuary, followed by, results from my 2007 work on invertebrate export and availability from three wetland systems.

16 Figure 1.1. The four life stages of chironomids. Emergence occurs as the pupa reaches maturity and moves to the surface of the water where it then transitions to the adult stage. Image courtesy of North Carolina University Cooperative Extension, Department of Entomology. 9

17 Chapter 2. Spatial and Temporal Patterns in Insect Emergence, Russian Island, Columbia River Estuary 10 Introduction Aquatic insects play a prominent role in the consumption and processing of primary production and associated detritus, and serve as an important food source for higher trophic levels, including a large number of fish, invertebrate, and avian species (Simenstad et al. 1982, Stagliano et al. 1998, Lott 2004). Studies of invertebrates in wetlands have increased in recent years; however, the number of such investigations remains relatively low when compared to those of rivers and streams, and lake communities (Stagliano et al. 1998, Williams and Williams 1998b, Mackenzie and Kaster 2004). With few exceptions (Diaz 1994, Hansen and Castelle 1999, Strayer and Smith 2000, Williams and Hamm 2002), studies of tidal freshwater insect assemblages are rare. Additionally, almost all of such studies treat insects at broad taxonomic levels, which consequently may limit interpretation of population dynamics, habitat associations, and seasonal patterns of abundance (Williams and Williams 1998a, Lott 2004, Gray 2005). Here I investigate the spatial and seasonal patterns of emergent insects within a freshwater tidal marsh of the Columbia River estuary. Within the lower Columbia River estuary, tidal freshwater wetlands serve as an ecologically significant connection in the landscape for juvenile salmon migrating to the marine waters of the Pacific Ocean. Ocean-type Chinook salmon are considered the most estuarine dependent, moving downstream immediately or soon after emergence and commonly rearing for several weeks within coastal wetland systems (Healey 1982, Simenstad et al. 1982, Healey 1991, Levings 1994, Bottom et al. 2005). This habitat reportedly offers abundant feeding opportunities, refuge from marine predators, and a physiological transition zone where juvenile fish may acclimate to saline waters (Simenstad et al. 1982, Thorpe 1994, Bottom et al. 2005). Examination of juvenile Chinook diets from marsh habitats has demonstrated the significant importance of wetland insects, particularly chironomids and other dipteran flies as food (Simenstad and Cordell 2000, Lott 2004). Dipterans, trichopterans and other marsh insects are shown to

18 11 be almost twice as energy rich as crustacean prey, such as the amphipods Corophium spp. and Eogammarus spp. (Gray 2005), further highlighting their importance to developing juvenile fish. Researchers with the National Oceanic and Atmospheric Administration (NOAA) and University of Washington have monitored juvenile Chinook habitat use and feeding in shallow tidal channels of the lower Columbia River estuary from 2002 to present. These studies consistently showed that emergent chironomids are the predominant prey item, with Indices of Relative Importance (IRI) as high as 90% (Lott 2004). Beyond shallow-water wetlands, marsh associated insects provide an important resource for juvenile Chinook in the main-stem estuary (Bottom et al. 2008). Their presence in the diets of these fish potentially signals the transfer or export of organisms to estuarine areas away from where they are produced. For aquatic insects, the metamorphosis, or emergence, to adult form is a switch from the benthic period of growth to the terrestrial reproductive and dispersal stage. After reaching maturity, the pupae begin the process of emergence by moving upward to the water s surface, either through the water column or by crawling along vegetation. It is during this period that the insect is most susceptible to predation (Oliver 1971). Lott (2004) describes this particular emergent life stage as the dominant life history stage appearing in the diets of juvenile Chinook in shallow water wetland habitats of the Columbia River estuary: the presence of immature wings in the stomach in addition to the small percentage of the diets comprised by terrestrial insects suggests that the juvenile Chinook frequently fed on the emerging pupa near the surface of the water. A number of methods are available for sampling adult insects, from fall-out traps (which are generally used to assess prey resources in the Columbia River estuary), to sweep nets and light traps. These traps collect insects from an open environment and are not effective methods for sampling the water column where salmon forage or collecting emergent chironomids. While fall-out traps deployed in wetland habitats can collect a large number of specimens, the traps may contain a high proportion of terrestrial species or individuals exported from various regions of the main-stem estuary. Therefore, the samples collected prior to this study offered little information about habitat associations or absolute numbers of prey directly available to juvenile salmon. Alternatively, an

19 12 emergent trap is designed as a truncated plastic cone, enclosing a defined area and collecting only the insects that emerge within that space. This allows for direct information on their point of origin and density, and offers a better representation of the prey juvenile Chinook will encounter and consume in tidal channels. This study reports on the seasonal and fine-scale spatial patterns of aquatic insect emergence, with emphasis on chironomid assemblages, from Russian Island, a freshwater tidal marsh of the lower Columbia River estuary. A vertical gradient exists along tidal channels, corresponding to the frequency and duration of tidal flooding, the composition of vegetation assemblages, and accessibility of wetland channels to fish (Simenstad et al. 2000). Due to infrequent tidal inundation and significant vegetation density, only a few fish species (threespine stickleback for example) are reported to utilize the extensive intertidal marsh surface (Kneib 1984, Simenstad et al. 2000). This study identifies how variation in within-system microhabitats contributes to insect production, improving researcher s assessment of the capacity of this wetland to support juvenile Chinook salmon. Because chironomids are highly diverse and difficult to identify, published surveys rarely categorized individuals at taxonomic levels finer than family and often ignore the rich information content for this ubiquitous and ecologically prominent group. Here I interpret habitat associations to family and genus levels to determine the richness of emergent insect assemblages and the contribution of individual taxa to observed patterns of emergent insect abundance. The primary objective was to provide new data on the spatial and temporal patterns of emergent insect and chironomid assemblages. Specifically, I: 1) determine temporal variations in emergent insects, particularly chironomids, in an emergent estuarine wetland; 2) quantify microhabitat associations of emergent insects as defined by gradients in tidal elevation and vegetation assemblage; and, 3) establish a baseline record of the chironomid fauna at the Russian Island marsh. These results are part of a comprehensive investigation of salmon-habitat associations in the Columbia River estuary to address basin-wide declines in Chinook salmon populations and life history diversity (Bottom et al. 2005).

20 13 Methods Study Area and Sites Russian Island, approximately 7 km 2, is located in Cathlamet Bay, south of the Columbia River s main shipping channel within the lower estuary (Fig. 2.1). This area has a temperate marine climate typical of the coastal Pacific Northwest region. The island is a freshwater tidal marsh located above the upstream limit of saltwater intrusion and characterized by a network of high-order dendritic channel systems. A complex, multi-assemblage emergent marsh, Russian Island is characterized by herbaceous wetland plants (Elliot 2004). The high marsh is dominated by Carex lyngbeii (Lyngby s sedge) and Myosotis laxa (small-flowered forget-me-not), with Oenanthe sarmentosa (Pacific water parsley), Potentilla anserine ssp. Pacifica (Pacific silverweed), Lythrum salicaria (purple loosestrife), Sagittaria latifolia (wapato), and Lotus corniculatus (birds foot trefoil) also commonly encountered. The low marsh is dominated by Polygonum hydropiperoides (water pepper), and the channel bottom is generally bare. Insect emergence was sampled from three dendritic channel systems located on the interior, southeast side of Russian Island (Fig. 2.1). I sampled at three locations per dendritic channel system, beginning near the point of origin (1 st order channels) and spaced with progressive increase in channel order to second and third order segments. This coincided with an increase in channel width, from less than 1 m to approximately 10 m. At each location, I sampled three distinct microhabitats along the tidal gradient: the channel base, low marsh bench, and high marsh. These microhabitats were defined subjectively as visually distinct units of geomorphology and vegetation along the channel, characterized by a slight change in elevation (range of about 1 m). In addition to distinct vegetation assemblages, each position varies in frequency and duration of flooding, associated water velocity, and sediment structure. Data Collection To describe prey availability and assess distributional patterns of emergent insects I sampled with an emergent trap as described by Davies (1984). I perceive the trap working analogous to a juvenile salmon foraging in the channel, by intercepting insects in

21 14 the process of emerging from aquatic to terrestrial environments. The emergent trap is designed as a truncated cone constructed from clear plastic sheeting with a modified water bottle attached to the top to contain emerged insects. The trap is weighted and placed on the ground, enclosing a basal area of 0.6 m 2 and capturing all of the insects that emerge within that area. To ensure the traps remain stationary, they were each tethered to two PVC pipes driven into the sediment. This design allows for measurement of numerical density as well as providing precise habitat of origin information. Similar devices have been successfully used to sample insect emergence in lake (Davies 1984), stream (Drake 1985) and freshwater wetland systems (Whiles and Goldowitz 2001, MacKenzie and Kaster 2004). Emergent insects were collected semi-monthly from April June, and monthly in July and August 2006, with traps deployed for approximately 48- hours at each event. The seasonal timing of insect collection overlapped with the greatest densities of out-migrating juvenile Chinook salmon (McCabe et al. 1986, Lott 2004). First order channels are considerably narrow without a well-developed low bench. Therefore, only the channel base and high marsh was sampled in the first order sections. This resulted in eight samples per channel, a total of 24 for each sample date. To facilitate the comparison of emergent traps with previous sampling methods and to provide additional interpretation of past analysis on Chinook foraging ecology (e.g. Lott 2004), fallout traps were used concurrent with emergent sampling. Insect fallout traps are designed to catch flying or terrestrial insects that may fall onto the water surface of the channel. They consist of a plastic bin (50 cm x 35 cm x 14 cm) filled to approximately half depth with soapy water, which will coat an insect s wings and inhibit it from escaping. One fallout trap was placed in the high marsh next to the emergent trap at the three channel orders sampled. All samples were sieved in the field through a mm sieve, preserved in 70% isopropanol, and then returned to the lab for processing. A dissecting microscope was used to sort, identify, and count collected specimens at the family level. Chironomids were then separated into morphospecies with select reference organisms slide mounted and identified under a compound microscope to genus. Identifications were made following Wiederholm (1989), and verified by L. Ferrington of the University of

22 15 Minnesota Chironomid Research Group. Keys for adult chironomid identifications are largely based on male morphology. Therefore, females were not slide-mounted, but were identified as specifically as possible according to their association with and/or resemblance to male specimens. A unique reference identification number was given to morphospecies that I was not able to identify; all others are referred to by their generic designation. Data Analysis Shifts in distributional patterns and abundance peaks over time were described qualitatively through graphical depictions for emergent insect families and chironomid genera. Tables listing proportional composition of taxa for both trap types (emergent and fallout) were comprised for each sample period to evaluate differences in sample methodologies. Due to the non-normal distribution of data, the nonparametric Kruskal- Wallis test (α =.05) was used to compare total insect abundance between microhabitats. Q-type multivariate analyses were used to distinguish assemblage patterns by date and emergence location under the basis of a priori knowledge of the microhabitat types. Preliminary analysis found no effect of channel or channel order. Therefore, for each sample date, abundance counts were pooled within a channel by microhabitat, with the three channels serving as replicates. Counts were converted to density (number per square meter) for the 48-hour period, then log transformed prior to analysis to reduce the variation within the dataset and provide a better representation in multivariate space. Taxa were considered rare and removed from analysis if they occurred in less than 5% of all samples. Two resulting data matrices were obtained with samples by date and microhabitat as objects (n=72) and insect taxa as descriptors. The first matrix organized all emergent insects at the family level (n=20); the second contained chironomids distinguished to the genus level (n=17) in order to view that group at a finer resolution. Using Bray-Curtis similarities, non-metric multidimensional scaling (NMDS) was performed to distinguish patterns of assemblage structure within the single dataset. This iterative procedure constructs a map of all samples to satisfy the conditions imposed by the similarity matrix. Spatial separation of sample points indicates similarity, so that

23 16 those closer together are more similar in community composition than those further apart. I used the Bray-Curtis similarity coefficient because it is considered highly appropriate for abundance data that is not normally distributed and contains a high frequency of zero values (Clarke and Warwick 2001). Analysis of Similarity (ANOSIM) with 999 permutations, tested for assemblage differences over time and among microhabitats. ANOSIM produces a global test statistic, R, ranging from 0 to 1, which indicates whether differences exist among groups. Large values are indicative of strong separation, where R is close to zero if the null hypothesis is true and similarities among and within groups are the same on average. When ANOSIM results were significant, I added pair-wise comparisons between specific pairs of groups to evaluate where the assemblage differences were actually occurring. Finally, the similarity percentages (SIMPER) analysis was used to determine species responsible for group distinctions. All data were analyzed using the Primer v.6 software package. Additionally, the Shannon-Wiener Index (H ) was used to compare values of taxonomic diversity among habitat features and over time, H = - P i log P i where Pi = the proportion of species in the community (Zar 1999). In the comparison, a higher H value corresponds to greater diversity. Results Insect Assemblage Structure, Densities, and Phenology Thirty-seven insect families were collected in emergent traps from the Russian Island tidal channels. Individuals from the Lepidoptera, and Trichoptera were not keyed to family. Non-Insecta taxa that were collected in emergent traps included Acari (mites), Araneae (spiders), and Corophium spp. Some insects that do not have an emergent life stage were still found in emergent traps, including the abundant Delphacidae. These taxa were not included in statistical analysis. Five families numerically dominate the assemblage, representing approximately 78% of total numerical abundance. Chironomidae were typically the most abundant family collected, accounting for 5-72% of the emergence over individual sample dates; the other dominant families included the

24 17 dipterans Dolichopodidae (0-36%) and Ephydridae (<1-50%), the hemipteran Delphacidae (0-29%), and the hymenopteran Braconidae (0-10%) (Fig. 2.2, Appendix A). Chironomids constitute the majority of emerging insects through spring months and peak in mid-june with an average of 25 individuals m -2. The total insect community shows a continued rise in abundance through August along with warming water temperatures and increase in the biomass of the island s vegetation (Fig. 2.2). The high densities of Dolichopodidae in July and August, and Delphacidae in August, contributed substantially to the increase in counts after chironomids had begun to decline. The NMDS plot shows a transition in the emergent insect assemblage over the season with samples forming early (April), mid- (May and June) and late (July and August) season groups (Fig. 2.3). An ANOSIM test confirmed these distinctions, with emergent assemblages found to be significantly different between months (R=0.792, p=0.001), and the lowest distinctions between May and June (R=0.233) and July and August (R=0.290). Monthly density trends for major taxonomic groups are consistent between emergent and fallout insect traps, with the exception of a much steeper decline in chironomid fallout densities in July (Fig. 2.4, Appendix B). Spatial Distribution of Emergent Insects The Kruskal-Wallis comparison of means indicated no significant difference in log-transformed abundance counts for emergent insects across the three microhabitats (p=0.483). However, the low bench was often the site of the highest rate of emergence, largely driven by the occurrence of abundant brachyceran flies (Fig. 2.5). Overall, chironomids were not significantly different in their microhabitat distribution (p=0.805), however other groups do appear to be associated with particular locations (Fig. 2.6). Differences in grouping of insect assemblages by microhabitat seen in NMDS plots (Fig. 2.7) were significant between all pairs of microhabitats when analyzed as a two-way crossed ANOSIM, accounting for known temporal shifts in the composition (R=0.669, p=0.001) (Table 2.1). Discounting taxa that were only collected from a single sample, just two emergent families were found exclusively in one microhabitat: Carabidae

25 18 (Coleoptera) from the high marsh and Saldidae (Hemiptera) from the low bench. None of the major families were restricted to one location; however, greater average abundances suggest habitat preference by certain taxa, driving distinctions in ensembles (Fig. 2.6, Appendix C). Chironomids represent the most abundant family in this survey and, consistent with their ubiquitous nature, are shown to occur somewhat uniformly over the microhabitat types. The SIMPER analysis identified the chironomid group as the greatest contributor to the similarity within each microhabitat (Appendix D). Therefore, while assemblages were shown to vary between these locations, an abundant chironomid community characterizes them all and no pattern of habitat associations can be determined for the family at this level of taxonomic resolution. Chironomidae Assemblage Structure, Densities, and Phenology Three of the 11 chironomid subfamilies were collected during the study period, Orthocladiinae (63.2%) being most abundant, followed by Chironominae (33.2%), and Tanypodiane (1.5%). Orthocladiinae dominated spring densities with rising water temperatures, while Chironominae are most abundant during summer months coinciding with the maximum water temperature for the year (Fig. 2.8). Seventeen genera were identified, with an additional six morphospecies separated, resulting in a minimum estimate of 23 genera present at Russian Island (Appendix E). Temporal patterns of the family are derived from the discrete timing of individual taxa, particularly the six major genera, which together account for 83.5% of total abundance. Furthermore, three genera (Orthocladius (Orthocladius), Thalassosmittia marina, and Paratendipes albimanus) strongly dominate, accounting for 68.4% of total density. In view of their abundance peaks, a notable temporal separation is seen among these genera (Fig. 2.9). Spatial Distribution of Chironomidae Distributional patterns, that are essentially averaged when the more specialized component species are grouped together, are apparent when the family is treated at a finer taxonomic resolution. When abundance data is analyzed at the generic level, results of a

26 19 two-way crossed ANOSIM, accounting for variation across sample dates, indicate significant differences in chironomid assemblages among microhabitats (R=0.457, p=0.001). Comparisons of specific pairs of the microhabitats find the high marsh chironomid assemblage to be significantly distinct when compared to the low bench and channel bottom assemblages. However, we fail to reject the null hypothesis for assemblage differences between the low bench and the channel bottom (Table 2.1). The SIMPER results identify the three dominant genera as driving assemblage distinctions, each characterizing a particular microhabitat (Table 2.2). The group s habitat associations, coupled with their individual emergence timing, results in peak chironomid abundances to vary over microhabitats through the season (Fig. 2.10). Densities of Orthocladius, Thalassosmittia, and Paratendipes, overlaid on an MDS plot of all samples, further demonstrate the minimal overlap in occurrence both spatially and temporally among these genera (Fig. 2.11). Total Numerical Emergence By plotting daily rates of emergence and extrapolating between sample points, total numerical emergence is then calculated as the area under the curve between the first and last sampling days (Davies 1984) (Fig. 2.12). Calculating the integral of daily emergence over the four-month sample season (April 10 August 8), I report a total of 5,804 emergent insects m -2 (densities averaged over the three microhabitats) (Table 2.3). The low bench shows the highest rate of emergence at 9,169 individuals m -2, largely driven by the high densities of dolichopodids. Averaged over the three microhabitats, chironomid total seasonal emergence is estimated at 1,524 individuals m -2 (Table 2.3). Taxa Diversity Measures of diversity over time for all emergent insects are shown to increase through the season with highest values in July and August. Chironomids have two peaks in diversity (late May and August), each corresponding to the absence of a peak emergence occurrence by one of the three dominant genera (Fig. 2.13). Of the 37 insect families identified, 33 occurred in the high marsh, and 27 in the low bench and channel

27 20 bottom. From the total 23 chironomid taxa, 20 occurred in the channel bottom, 18 in the low bench, and 16 in the high marsh. Following the patterns of species richness, diversity was greatest in the channel bottom for chironomids and in the high marsh for all insects (Fig. 2.13). Discussion This study describes a transitional insect assemblage present over the period of juvenile Chinook outmigration and rearing in dendritic channels of a freshwater marsh and conclusively demonstrates the emergence of important insect prey from this habitat. With a mean tidal range of 1.92 m, water routinely floods the marsh plain at Russian Island then recedes to completely dewater the smaller, lower order channels. This periodic alternation between inundation and atmospheric exposure results in a unique and complex ecosystem. Juvenile salmon tend to forage from the low bench and channel bottom microhabitats, areas I will refer to as low-marsh habitats throughout the discussion. It is presumed that the fish avoid moving into the shallow, densely vegetated high marsh when that area is flooded, especially late in the season when the vegetation forms a thick barrier; however, there are few data that isolate use of specific marsh features (Simenstad et al. 2000). Changes in the composition of insect assemblages occur quickly in space and time, providing a sensitive indication of variation in ecosystem conditions, yet also making interpretation of community patterns particularly susceptible to decisions on the scale of observation. When we narrow our scale of spatial, temporal, or taxonomic interest, distinct patterns may become more apparent, though often at the expense of what is considered regular statistical behavior (Levin 1992). For instance, this study is limited to a fraction of Russian Island s total area. Exposure to different erosion and accretion processes throughout the island created a distinct age stratification of the marsh ecosystem with associated variations in elevation, soil texture, organic matter, and plant assemblages (Elliot 2004). Such variables are known to affect densities of benthic macroinvertebrates (Franquet 1999, MacKenzie and Kaster 2004) and consequently would influence adult insect emergence because

28 21 emergence is a function of larval distribution (Stagliano et al. 1998). Expanding the extent of sampling to include marsh assemblages of divergent development stages would likely increase the reported number of taxonomic groups present. Therefore, this study provides a minimum estimate of emergent insect families and chironomid genera within Russian Island. I sampled insects monthly or semi-monthly over a 48-hour period. Considerable variation occurred between sample months, reflecting the ephemeral nature and swift turnover of insect populations and suggesting that the monthly sampling interval could have missed some intervening emergence events. Comparison of Trap Methodology Emergent traps at Russian Island collected 37 insect families from emergent traps (including some non-emergent families), slightly more than the 33 insect families sampled with the fallout traps. Families found in the emergent, but not the fallout, traps, included: Corixidae (Hemiptera), Dytiscidae (Coleoptera), Empididae (Diptera), Haliplidae (Coleoptera), Megaspilidae (Hymenoptera), Platygastridae (Hymenoptera), and Psyllidae (Diptera). Those occurring in the fallout, but not the emergent, traps, included: Halictidae (Hymenoptera), Reduviidae (Hemiptera), and Sciomyzidae (Diptera). Of particular interest was the near absence of Sialidae (n=2) and Coenagrionidae (n=1) from emergent traps. Both of these taxa are common emergent aquatic insects and were collected in fallout traps between May through July (Sialidae) and June through August (Coenagrionidae). This suggests these large-bodied families may avoid emergent traps, which has also been reported from other studies (Stagliano et al. 1998, Whiles and Goldowitz 2001). Abundances of insect groups compared among samples from emergent and fallout traps showed similar trends, implying the latter may be a suitable indicator of relative insect prey availability to juvenile Chinook feeding in the tidal channels. However, given the variation in assemblages between microhabitats, the emergent trap does offer the advantage of examining small-scale habitat associations and limiting collections to microhabitats presumably utilized by salmon.

29 22 Dominant Insect Families Dipteran families, particularly Chironomidae, and the Brachyceran Dolichopodidae and Ephydridae, numerically dominated emergence from Russian Island. The dominant taxa reported here are nearly identical to those from a study of insect emergence conducted in marsh habitats of the Fraser River estuary, British Columbia (Whitehouse et al. 1993). That study did find chironomids to constitute a much higher numerical proportion of the total emergence, ranging from 40 to 75%, compared to the approximate 27% from Russian Island. The study from Fraser River, conducted in the late 1980s, represents the one comparable survey of chironomid emergence from estuaries of this region. The maximum daily density (m -2 ) of emergent chironomids reported in this study was 340 individuals collected from the riparian or high intertidal zone (Whitehouse et al. 1993). Compared to samples from Russian, this density was approximately twelve times greater than the daily maximum of chironomids m -2 emerging from the high marsh in mid June (Fig. 2.10). Unfortunately, the Fraser River study does not include chironomid identifications beyond the family level, and consequently a comparison of assemblage structure or species richness was not possible. Densities of emergent dolichopodids at Russian Island were significantly greater from the low bench than from the other microhabitats and were very high for the months of July and August. Over these two months, dolichopodids emerged at the highest densities compared to all other insects from the whole season. Ephydrids, which are adapted to living in brackish environments (Foote 1995), were more cosmopolitan in their distribution, although they were found over 80% of the time from low marsh habitats. Both of these stout-bodied flies are consumed by juvenile Chinook (Lott 2004) and represent a considerable amount of biomass production particularly when compared to the smaller Nemotecera flies. While the Hemipteran family Delphacidae was very abundant in August samples, constituting nearly 30% of total density (over 50% of total density from the high marsh), they are terrestrial insects that undergo incomplete metamorphosis and are not considered emergent. All species of delphacids are phytophagous. At Russian Island over 90% of all delphacids occurred in the densely vegetated high marsh during August when plants were at peak production. During their

30 23 peak summer abundance, hemipteran species also contribute to juvenile Chinook diets (Lott 2004). Chironomidae Assemblage Twenty-three chironomid genera from three subfamilies were collected with emergent traps at Russian Island. The subfamily Chironominae was the most species rich, but Orthocladiinae was most abundant. Many Orthocladiinae species are adapted to cool, well-oxygenated water and are generally associated with upland streams (Pinder 1995). Chironominae are characteristic of large lowland rivers, although taxa of the Orthocladiinae group may thrive in areas such as Russian Island, where aquatic vegetation exists (Pinder 1995). The chironomids of the Pacific Northwest remain poorly known, with very little published on the regional fauna except for the papers of Morley and Ring (1972a, 1972b). The lack of similar surveys prevents comparison of faunal richness in this study with that of other sites in the region. A broad survey of benthic invertebrates in the Fraser River basin in British Columbia identified 71 chironomid genera. However, the large sampling extent (i.e. throughout the entire Fraser watershed) limits direct comparisons to my fine-scale study. Reports from other regions show 31 chironomid genera identified from a small Alabama wetland (Stagliano et al 1998), 32 from an Alabama stream (Reynolds and Benke 2006), and 25 from three small prairie ponds in Central Saskatchewan (Driver 1977). When sampled comprehensively, chironomid richness will often approach 80 or more species per site (Ferrington 2008). The level of identification of any macroinvertebrate group is often determined by the objectives of the study. General trends related to the spatial and temporal patterns are seen at the family level, but these overall patterns are clearly derived from the more specialized individual timing and requirements of component species. The most abundant chironomid genus at Russian Island was Orthocladius (Orthocladiinae). These chironomids emerged at low numbers early in the season but spiked in the high marsh when emergence rates reached approximately 43 individuals m -2 d -1. Other abundant genera included Paratendipes (Chironominae), which were associated with the channel bottom and emerged from May through July, and Thalassosmittia (Orthocladiinae),

31 24 which emerged largely from the low bench between April and early June. Thalassosmittia is a known intertidal chironomid and was described in detail from the west coast of Canada under the previous generic name Saunderia (Morley and Ring 1972a, 1972b). Habitat associations of the three dominant genera at Russian Island result in chironomid abundance peaks to shift in space over time. Any attempt to interpret seasonal patterns of chironomids must consider the strong influence of individual taxa on total emergence (Stagliano et al. 1998). For example, the mid-june peak in abundance for the family is solely attributed to the dramatic rise in Orthocladius emergence at that time. Similarly, when combined, the divergent spatial patterns within the chironomid family are essentially averaged, masking distinctions and resulting in a fairly uniform distribution. The NMDS plot of chironomid density (Fig. 2.12) reveals temporal and spatial differences between samples that distinguish emergence patterns of the dominant genera. A stress level of 0.22 associated with this plot implies potential error in the model, and therefore, relative differences between objects should be interpreted cautiously (Clarke and Warwick 2001). Consistent with that found in the literature (Bass 1986, Franquet 1999), the three dominant chironomid genera, as well as some of the common insect families, do show preference among the microhabitats, but not specialization. By not acting as specialists, chironomids are able to adapt to a variety of ecosystems and conditions, characteristics that are consistent with the ubiquitous nature of the family (Cranston 1995, Ferrington 2008). Heterogeneity within a habitat often results in small-scale spatial patterns in invertebrate distributions (Kneib 1984). While environmental variables were not measured directly for this study, the difference in elevation between microhabitats coincides with variation in the frequency and duration of tidal (and riverine) flooding, associated water velocity, and vegetative assemblage. While the ANOSIM test showed that all microhabitats were composed of distinct insect families, it failed to detect a difference in chironomid assemblages between the low bench and channel bottom sites. Because these low marsh microhabitats experience similar tidal exposure, flow, and vegetative conditions, the similarity of chironomid assemblages is not surprising. Kneib

32 25 (1984) also demonstrated zonal distribution patterns along a tidal gradient in salt marsh channels for many numerically dominant invertebrates. Total numerical emergence Averaging over the three microhabitats, the integral of daily emergence over the sample season was approximately 5,800 insects m -2. Other emergent insect studies have reported total annual densities, such that my estimate of total seasonal emergence provides a conservative comparison. While emergence is expected to be low over winter months, sampling at Russian Island ceased before insect abundances began to decline. Regardless, the rate of seasonal emergence reported above is greater than or comparable to that from a number of other wetland sites (Table 2.4). A more appropriate comparison would be to studies quantifying insect emergence in areas subject to tidal forcing. Unfortunately, such data are rare. I used the integral daily emergence technique to estimate total emergence for the low-marsh area of the north tidal channel sampled at Russian Island. With a known area of 4,185.9 m 2, the low-marsh insect emergence over the four-months totaled 26,198,260 individuals. The integral of daily chironomid emergence over the sample season was approximately 1,500 chironomids m -2, comparable to annual chironomid densities reported from lakes in Ontario, Canada (Rosenberg et al. 1988) and headwater streams of central Oregon (Banks et al. 2007), but low compared with results from freshwater nontidal wetlands (Mizuno et al. 1982, Wrubleski and Rosenberg 1990, Stagliano et al. 1998). Chironomids had the lowest diversity and total density in the Russian Island high marsh, an area that may be unavailable to juvenile Chinook. Again, by extrapolating integral daily emergence to the area of one channel, we estimate that 6,800,000 chironomids emerged from the low-marsh over the sample season. Conclusions Despite their apparent ecological significance, estuaries have experienced dramatic loss and degradation of salmon habitat. The Columbia River estuary has

33 26 undergone significant changes following the rapid growth of human civilization and an estimated 77% of tidal marshes and 62% of tidal swamps have been lost in relation to the historic (Thomas 1983). Focusing on the recovery of ecosystem processes that promote and sustain shallow-water habitats and their related elements may potentially facilitate juvenile salmon recovery. An appreciation of small-scale physical complexity and its role in the maintenance of persistent invertebrate richness and population stability should be acknowledged in the design and management of restoration projects. Additionally, knowledge of wetland insect communities is something that will increase in importance as restoration ecologists realize their importance as evaluative criteria (Lammers- Campbell 1998). For macroinvertebrates, it is expected that the strongest spatial correlations occur at small scales, between assemblages and their habitat, because this is the template experienced directly by the organisms, which are relatively immobile as larvae (Pik et al. 2002). For a given date, insect composition and abundance was consistent within each microhabitat, supporting the notion that this scale is appropriate for examining local patterns in the spatial distribution of aquatic insects. It is within this fairly narrow area that chironomids and other macroinvertebrates perform basic life history functions (feed, find shelter, and move about), consequently localizing insect emergence (Davies 1984, Franquet 1999, Baxter et al. 2005). These distinctions may drive decisions of microhabitat use and prey selection for salmon feeding in the channel (McIvor and Odum 1988). While the contribution of chironomids to the diets of juvenile Chinook salmon is well documented for these freshwater tidal marshes (Lott 2004), this is the first study to describe the temporal and spatial distribution of chironomids throughout these habitats. Within-family differences in microhabitat preference were detected at the generic level, indicating the importance of ecosystem complexity. This improves our understanding of the marsh s capacity to support juvenile Chinook by specifically assessing availability of the particular life stage of prey targeted by these consumers within the particular channel areas utilized by the fish. A logical extension of this research would be to examine the taxa-specific selection by juvenile Chinook salmon within the chironomid family. Based

34 27 on an assessment of prey availability using insect fallout traps, Chinook were shown to select for chironomids among the other invertebrate resources in the Columbia River estuary (Lott 2004). With an understanding of the variation in distribution shown by this taxonomically diverse assemblage, selection within the family may demonstrate distinctions in microhabitat use by juvenile salmon within tidal channels. Because chironomids are often extremely abundant and difficult to identify, most studies have grouped them at broad taxonomic levels (Williams and Williams 1998a, Gray 2005, Reynolds and Benke 2006). Yet, as this survey indicates, taxonomy is fundamental to our ability to interpret biological changes and to draw inferences about ecosystem functions and the current state of aquatic environments. Not only does this study provide a baseline record of chironomid fauna, it expands our knowledge of the basic ecology of these groups by reporting on emergence phenologies and microhabitat preferences. The diverse chironomid assemblage present over the study period implies a consistent, if not varied, source of food for salmon feeding in tidal channels.

35 28 Russian Island Figure 2.1. Counter clockwise from the top: Columbia River estuary; Russian Island, box outline locating sampled tidal channels; 3 tidal channels with sample locations marked. Microhabitats were sampled at each location.